1. Field of the Invention
The present invention relates to a drive circuit for a fluorescent display tube, more particularly to a drive circuit in which each of the high withstand-voltage output transistors for turning on and off the voltages of the grids and/or segments of a fluorescent display tube is divided into a plurality of transistor elements, thereby preventing the output transistors from being destroyed by excessive voltage.
2. Description of the Prior Art
In a fluorescent display tube used for displaying numerals, characters, and the like, a high withstand-voltage output transistor is provided for and connected to one or more segments and grids. A required numeral, character, or the like is displayed by selectively controlling the voltage of each segment or grid using these transistors.
These high withstand-voltage transistors output, for example, voltages of +5 V to -25 V. When the voltage of a segment or grid is changed, from +5 V to -25 V, an excessive voltage of, for example, near -50 V is applied to the drive transistor for the grid or segment due to the capacitance between the segment and the grid. This excessive voltage can destroy the drive transistor.
In order to prevent the destruction of a drive transistor due to excessive voltage, conventional circuits have been provided with a diode connected between the output terminal of each drive transistor and the power source. The conventional circuits, however, are unsuitable when fluorescent display tubes are driven and controlled by, for example, a one-chip microcomputer or by other circuits. The diodes necessitate a relatively large space in the peripheral circuit, decreasing the packing density of the system and complicating the manufacturing process, thereby increasing costs.